1. Field of the Invention
This invention relates to an active matrix liquid crystal display panel of a structure wherein liquid crystal is held between a pair of transparent insulating substrates having thin film field effect transistors and electrodes thereon and also to a wiring designing method for the active matrix liquid crystal display panel.
2. Description of the Prior Art
An active matrix liquid crystal display panel (hereinafter referred to as AMLCD) wherein a thin film field effect transistor (hereinafter referred to as TFT) is used as a switching element for a pixel has a high picture quality and is applied widely to display devices for portable computers, light bulbs for projection type display devices and like devices.
Generally in an AMLCD, liquid crystal is held between a TFT substrate of a structure which includes scanning lines, signal lines and pixel electrodes connected to thin film transistors disposed in the proximity of intersecting points between the scanning lines and the signal lines and an opposing substrate, and controlled voltages are applied between pixel electrodes and opposing electrodes to control amounts of transmission light through corresponding pixels. The AMLCD is most characteristic in that the potentials between the electrodes between which the liquid crystal is held are easy to control comparing with those of a simple matrix type liquid crystal display device and a display of a high quality superior in contrast and angle of visibility is obtained.
In recent years, in order to further improve the visibility angle characteristic, a display method called in-plane switching mode has been proposed (refer to Asia Display '96). According to this method, a pixel electrode of a comb-like shape is formed on a TFT for each pixel, and an opposing electrode having a similar comb-like shape is formed in the same plane. When a voltage is applied between the electrodes, a parallel electric field is formed in the plane of the liquid crystal layer and varies the direction of a director of the liquid crystal thereby to control the light transmission amount.
In this mode, since the director acts only in a direction substantially parallel to the plane of the liquid crystal layer, such a problem that the director is diverted from the plane of the liquid crystal layer as in a TN mode so that the relationship between the light transmission amount and the applied voltage is very different depending upon whether the liquid crystal display device is viewed from the direction of the director or from the direction of a normal to the liquid crystal layer does not occur. Consequently, an image which is substantially uniform as viewed from a very wide visual angle can be obtained.
FIG. 1 is a plan view showing a construction of a pixel of a conventional active matrix liquid crystal display panel, and FIG. 2(a) is a sectional view taken along line A-A' of FIG. 1 while FIG. 2(b) is a sectional view taken along line B-B' of FIG. 1.
Referring to FIGS. 1, 2(a) and 2(b), a plurality of scanning lines 103 and a plurality of signal lines 104 intersect each other on a main surface of first transparent insulating substrate 101 formed from a glass substrate or a like substrate so that the main surface is partitioned into gratings. In each of the gratings, a set of active pixel elements including a pixel TFT disposed in the proximity of an intersecting point between scanning line 103 and signal line 104 and pixel electrode 107 driven by this pixel TFT is disposed.
Referring to FIG. 2(a), gate electrode 102 formed integrally with scanning line 103 is formed on first transparent insulating substrate 101, and a channel part made of island-shaped amorphous silicon 109 is provided on gate electrode 102 with gate insulating film 115 interposed therebetween. Drain electrode 111 formed integrally with signal line 104 and source electrode 117 formed integrally with pixel electrode 107 are provided on island-shaped amorphous silicon 109, and further, protective insulating film 113 is provided in such a manner as to cover over the entire pixel to construct a pixel TFT.
As shown in FIG. 2(b), pixel electrode 107 and signal line 104 are formed in a same layer, and opposing electrode 114 opposing to pixel electrode 107 is formed in another same layer as that in which scanning line 103 and gate electrode 102 are formed. Opposing electrode 114 is formed integrally with opposing electrode bus line 105 formed in parallel to scanning line 103 so that a potential may be supplied thereto from the bus line. Each of pixel electrode 107 and opposing electrode 114 is formed in a comb-like shape so that in-plane switching is realized by an electric field produced in parallel to the liquid crystal layer between the two electrodes.
Although the active matrix liquid crystal display panel described above is superior in that a good display having a very wide angle of visibility can be obtained due to the in-plane switching mode structure, it has the following problems.
In such a conventional structure wherein an opposing electrode bus line for applying a potential to an opposing electrode is formed in a same layer as that of a scanning line as described above with reference to FIG. 2, if it has an arrangement (close arrangement) wherein the opposing electrode bus line is arranged very closely to the scanning line, then short-circuiting may possibly occur between the scanning line and the opposing electrode bus line because of a defect in patterning of the scanning line and the opposing electrode bus line. Consequently, the opposing electrode bus line cannot be arranged closely to the scanning line, and the numerical aperture (ratio of the effective display area formed between a pixel electrode and an opposing electrode which occupies in a repeat pattern for one pixel) cannot have a high value.
Further, it likely occurs that stray light of back light enters the channel part, whereupon an impurity in the liquid crystal layer acts on the TFT to increase the off current of it, resulting in failure in display. Therefore, the display reliability is not high.